Motor generator units that include an AC generator and an internal combustion drive engine are well known. Also known are generator output voltage control systems and also idle control systems that control the speed of the drive engine.
For example, commonly assigned U.S. Pat. No. 4,459,490, issued Jul. 10, 1984, entitled "Idle Control System for Generators" by George M. Brandon, describes the use of a speed responsive governor to maintain a relatively constant engine speed regardless of the electrical load being powered by the generator, except when the generator is unloaded. An idle control circuit detects the no-load condition and actuates a device, such as a solenoid or an electromagnet, to vary a throttle and reduce the speed of the drive engine to a predetermined idle speed. In the approach of Brandon a high sensitivity electronic switching element is rendered conductive upon the absence of current draw from the generator to complete a current path through an idle control electromagnetic coil.
In commonly assigned U.S. Pat. No. 4,697,135, issued Sep. 29, 1987, entitled "Electronic Voltage Regulator", also by George M. Brandon, there is described the use of a rectified and filtered representation of the output voltage of the generator, which is proportional to the output voltage and which has a ripple component, and also the use of a dc voltage that is proportional to the frequency of the generator output. A comparator compares these two voltages. The comparator includes hysteresis so that it switches on and off with the ripple voltage component. A signal produced by the comparator controls the application of current from a quadrature winding of the generator to the generator field so as to maintain the output voltage of the generator substantially constant.
More particularly, as the output voltage of the generator falls a duty cycle of a comparator increases. This increase in duty cycle, operating through a power stage, causes an increase in a generator field current and a consequent increase in the output voltage. Conversely, as the output voltage of the generator rises the duty cycle of the comparator decreases. The decrease in duty cycle, operating through the power stage, causes a decrease in the generator field current and a consequent decrease in the output voltage.
While the idle control system and the voltage control system disclosed in the foregoing U.S. Patents are well suited for their intended applications, it is one object of this invention to provide an improved idle and voltage control system for use in a motor generator.
The inventor has recognized that the lowering of the generator output voltage during idle periods is an important consideration. As may be realized, if the voltage is not lowered during this time the voltage regulator circuitry will attempt to maintain the generator voltage at its nominal level. However, since the speed of the drive engine is lowered during idle periods, the generating capability of the generator is reduced, resulting ultimately in the potential to apply large currents to the excitation winding of the rotor. This is generally unacceptable as it tends to overheat the rotor, due to the combination of high currents and a reduced airflow that results from the lower generator speed. An ability to reduce the generator output voltage during an idle period would beneficially eliminate the high rotor currents.
Another problem that the inventor has recognized relates to running the generator with an intermittent load. For example, if a drill were plugged into the generator and used intermittently, as would occur when a series of holes were being drilled, the generator would repetitively cycle between a loaded and an unloaded state. As a result, the idle control system would operate to speed up and slow down the drive engine in an attempt to respond to the loading and unloading of the generator. As can be appreciated, a rapid and prolonged cycling of the drive engine speed may be undesirable.
A further problem that has been recognized relates to a conventional voltage regulator circuit which monitors only one output line of the generator. A heavy load placed on the monitored line may cause the voltage on the un-monitored line to rise to high levels. Conversely, if a heavy load is placed on the un-monitored line, the voltage regulator will not react to the drop in that line's voltage. This may result in the induction motor starting capability of that line being compromised. These voltage extremes may also be damaging to any equipment that is connected to the un-monitored line.
Another problem recognized by the inventor relates to the starting of the generator. That is, in many cases it is required to provide a large quadrature or excitation winding to provide sufficient rotor excitation during start-up. However, providing a quadrature winding that provides a large output voltage can be detrimental during times of extreme overload, as it may adversely impact the ability of the generator output voltage to collapse at a correct rate.